1. Field of the Disclosure
The present disclosure relates to directional drilling and, in particular, to a method of determining an expected distribution of errors in downhole survey parameter measurements used in directional drilling.
2. Description of the Related Art
One aspect of direction drilling of a borehole in a formation includes determining a current position and/or orientation of a drill bit or other suitable unit of a drill string. Various survey instruments are often used to determine the current position and orientation. These survey instruments include a magnetometer for taking magnetic measurements in three orthogonal directions, an accelerometer for taking gravity measurements in three orthogonal directions and various tools for measuring toolface azimuth and inclination of the drill string. Various errors may arise during any of these measurements. The error that arises in survey measurements is assumed to conform to a pre-specified error model which may be used to predict borehole position uncertainty as a function of the survey errors. However, various factors, such as excessive magnetic interference from a bottomhole assembly of the drill string, magnetic effects of magnetic drilling mud, poor calibration of survey instruments, etc., may introduce errors that lie outside of the specifications of the error model.
Multi-station analysis is generally applied to a set of surveys as a method of quality control by estimating corrections to various error parameters such as magnetometer bias, scale factors, reference dip angle, etc. that may cause error in survey parameters. These estimated corrections may then be used to correct the survey parameters to conform to the error model. One problem of multi-station analysis is that while error models assume independent (uncorrelated) terms, in actuality various error parameters, for example z-axis bias and z-axis scale factor, become correlated and therefore generally may not be completely separable. The present disclosure provides a method of determining error terms that includes calculating the correlations between error terms.